Glass substrate for a magnetic disk and magnetic disk

ABSTRACT

Provided are a magnetic disk substrate and a method of manufacturing the same, wherein the magnetic disk substrate has very few defects present on its surface with an arithmetic mean roughness (Ra) at a level in the vicinity of 0.1 nm and thus is suitable as a substrate for a magnetic disk with high recording density. The magnetic disk glass substrate is such that the arithmetic mean roughness (Ra) of the main surface of the glass substrate measured using an atomic force microscope with a resolution of 256×256 pixels in a 2 μm×2 μm square is 0.12 nm or less and the number of defects detected to have a size of 0.1 μm to 0.6 μm in plan view and a depth of 0.5 nm to 2 nm is less than 10 per 24 cm 2 , wherein the defects are each detected using a shift in wavelength between incident light and reflected light upon irradiating and scanning helium neon laser light with a wavelength of 632 nm on the main surface of the glass substrate.

TECHNICAL FIELD

This invention relates to a glass substrate for a magnetic disk adaptedto be mounted in a hard disk drive device, and to the magnetic disk.

BACKGROUND ART

As a magnetic recording medium adapted to be mounted in a hard diskdrive device (HDD device), there is a magnetic disk. The magnetic diskis produced by coating a NiP film on a metal substrate made of analuminum-magnesium alloy or the like or by laminating a magnetic layerand a protective layer on a glass substrate or a ceramic substrate.Aluminum alloy substrates have conventionally been widely used assubstrates for magnetic disks. However, following the reduction in sizeand thickness and the increase in recording density of magnetic disks inrecent years, glass substrates excellent in surface flatness andthin-plate strength as compared with the aluminum alloy substrates havestarted to be used.

With respect to magnetic disks formed with at least a magnetic layer ona magnetic disk glass substrate, the increase in recording density hasadvanced year by year and those magnetic disks having a magnetic layercontaining granular particles are becoming predominant. In such amagnetic layer, it is necessary to reduce the particle size of thegranular particles or enhance the crystal orientation of the granularparticles in order to achieve a further increase in recording density(e.g. 160 GB or more per disk, particularly 250 GB or more per disk). Inorder to reduce the particle size of the granular particles or enhancethe crystal orientation of the granular particles as described above, itis necessary to improve the properties of a magnetic disk glasssubstrate, particularly to reduce its surface roughness or reducedefects present on its surface. As a magnetic disk glass substrate witha reduced surface roughness, there is, for example, one disclosed inPatent Document 1.

In recent years, in order to achieve a further increase in recordingdensity, there has been advanced the development of patterned media suchas a discrete track medium in which adjacent tracks are magneticallyisolated from each other. As a method of manufacturing such a patternedmedium, there is, for example, a method of forming a magnetic layer on aglass substrate and then physically dividing this magnetic layer tothereby isolate tracks from each other. When dividing the magneticlayer, a pattern is formed on the magnetic layer using the nanoimprinttechnique.

In this event, if a defect (particularly a convex defect) is present onthe glass substrate, the above-mentioned pattern is not formed on themagnetic layer where this defect is present. Specifically, the defect onthe glass substrate is succeeded in the formation of the magnetic layerso that the defect is also formed on the magnetic layer. If thenanoimprint is carried out in this state, a pattern of a stamper is notformed only around this defect. Further, depending on circumstances,there is a possibility of damage to the stamper. Therefore, in themanufacture of a patterned medium using the nanoimprint technique, it isrequired that defects be extremely few on a glass substrate.

Prior Art Document Patent Document

Patent Document 1: JP-A-2006-95676

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

When the surface roughness is at a very low level, for example, when thearithmetic surface roughness (Ra) is in the vicinity of 0.1 nm, reducingthe surface roughness and reducing defects present on a surface tend tobe in a trade-off relationship. That is, just because the surfaceroughness is reduced, it does not necessarily follow that the number ofdefects present on the surface is reduced. This is because, in the caseof a glass substrate having a surface roughness at the level in thevicinity of 0.1 nm, cleaning which is conventionally carried out for thepurpose of removing adhering matter and so on becomes a cause ofroughening a surface of the glass substrate. That is, this is because,in the case of the glass substrate having the very low surface roughnessat the level in the vicinity of 0.1 nm, it is necessary to use, in orderto maintain the surface roughness, a relatively weak chemical solutionin cleaning which is carried out for removing defects present on thesurface. This tendency is significant particularly when a magnetic diskglass substrate is made of a multi-component glass such as analuminosilicate glass.

This invention has been made in view of these circumstances and has anobject to provide a magnetic disk glass substrate which has very fewdefects present on its surface with an arithmetic mean roughness (Ra) ata level in the vicinity of 0.1 nm and thus is suitable as a substratefor a magnetic disk with high recording density, and to provide such amagnetic disk.

Means for Solving the Problem

A magnetic disk glass substrate according to one aspect of thisinvention is characterized in that an arithmetic mean roughness (Ra) ofa main surface of the glass substrate measured using an atomic forcemicroscope with a resolution of 256×256 pixels in a 2 μm×2 μm square is0.12 nm or less and, among defects detected to have a size of 0.1 μm ormore and 0.3 μm or less upon irradiating light with a wavelength of 405nm onto the glass substrate with a spot size of 5 μm and detectingscattered light from the glass substrate, the number of the defectsfixed to the glass substrate is 1 or less per 24 cm².

In the magnetic disk glass substrate according to one aspect of thisinvention, it is preferable that a ratio (Ra/Rp) of the arithmetic meanroughness (Ra) to a maximum peak height (Rp) on the main surface of theglass substrate is 0.15 or more.

A magnetic disk glass substrate according to another aspect of thisinvention is characterized in that an arithmetic mean roughness (Ra) ofa main surface of the glass substrate measured using an atomic forcemicroscope with a resolution of 256×256 pixels in a 2 μm×2 μm square is0.12 nm or less and the number of defects detected to have a size of 0.1μm or more and 0.6 μm or less in plan view and a depth of 0.5 nm or moreand 2 nm or less is less than 10 per 24 cm², the defects each beingdetected using a shift in wavelength between incident light andreflected light upon irradiating and scanning helium neon laser lightwith a wavelength of 632 nm on the main surface of the glass substrate.

In the magnetic disk glass substrate according to another aspect of thisinvention, it is preferable that a ratio (Ra/Rv) of the arithmetic meanroughness (Ra) to a maximum valley depth (Rv) on the main surface of theglass substrate is 0.15 or more.

In the magnetic disk glass substrate of this invention having theabove-mentioned structure has very few specific defects present on itssurface with the arithmetic mean roughness (Ra) at the level in thevicinity of 0.1 nm and thus is suitable as a substrate for a magneticdisk with a high recording density of 160 GB or more per disk,particularly 250 GB or more per disk.

In the magnetic disk glass substrate of this invention, it is preferablethat the glass substrate has a disk shape with a hole at a centerthereof and, assuming that a distance from the center to an outermostperiphery is 100%, a difference (Ra_(O)-Ra_(I)) between an arithmeticmean roughness (Ra_(O)) of the main surface in a range of 80% or moreand 90% or less from the center and an arithmetic mean roughness(Ra_(I)) of the main surface in a range of 10% or more and 20% or lessfrom the center is 0.01 nm or less.

In the magnetic disk glass substrate of this invention, it is preferablethat the glass substrate has the main surface and an end face, the mainsurface and the end face each have a compressive stress layer, and thecompressive stress layer of the main surface has a depth which isshallower than that of the compressive stress layer of the end face.

A magnetic disk according to this invention is characterized in that atleast a magnetic layer is formed on the magnetic disk glass substrate.In this case, it is preferable that the magnetic disk is a patternedmedium in which at least adjacent recording tracks are magneticallyisolated from each other.

A magnetic disk glass substrate manufacturing method according toanother aspect of this invention is characterized by comprising apolishing step of polishing a main surface of a glass substrate using apolishing liquid containing an additive and a cleaning step of cleaningthe glass substrate polished, using a cleaning liquid containing theadditive.

In the magnetic disk glass substrate manufacturing method of thisinvention, it is preferable that the additive contains at least one ofcarboxylic acid, polyvalent amine, amino acid, aminopolycarboxylic acid,phosphonic acid, phosphinic acid, phosphoric acid, pyrophosphoric acid,tripolyphosphoric acid, amino trimethylene phosphonic acid, and saltsthereof.

In the magnetic disk glass substrate manufacturing method of thisinvention, it is preferable that the additive is contained in thepolishing liquid in a range of 0.01 wt % or more and 10.0 wt % or lessand is contained in the cleaning liquid in a range of 0.01 wt % or moreand 5.0 wt % or less.

In the magnetic disk glass substrate manufacturing method of thisinvention, it is preferable that the additive is contained in thepolishing liquid in a range of 0.1 wt % or more and 5.0 wt % or less andis contained in the cleaning liquid in a range of 0.1 wt % or more and3.0 wt % or less.

Effect of the Invention

In the magnetic disk glass substrate of this invention, an arithmeticmean roughness (Ra) of a main surface of the glass substrate measuredusing an atomic force microscope with a resolution of 256×256 pixels ina 2 μm×2 μm square is 0.12 nm or less and, among defects detected tohave a size of 0.1 μm or more and 0.3 μm or less upon irradiating lightwith a wavelength of 405 nm onto the glass substrate with a spot size of5 μm and detecting scattered light from the glass substrate, the numberof the defects fixed to the glass substrate is 1 or less per 24 cm².Accordingly, the number of defects present on the surface with thearithmetic mean roughness (Ra) at the level in the vicinity of 0.1 nm isvery small.

Therefore, it is suitable as a substrate for manufacturing a magneticdisk having magnetic particles with a much smaller size, for example,having a recording density of 160 GB or more per disk, particularly 250GB or more per disk.

In the magnetic disk glass substrate of this invention, an arithmeticmean roughness (Ra) of a main surface of the glass substrate measuredusing an atomic force microscope with a resolution of 256×256 pixels ina 2 μm×2 μm square is 0.12 nm or less and the number of defects detectedto have a size of 0.1 μm or more and 0.6 μm or less in plan view and adepth of 0.5 nm or more and 2 nm or less is less than 10 per 24 cm², thedefects each being detected using a shift in wavelength between incidentlight and reflected light upon irradiating and scanning helium neonlaser light with a wavelength of 632 nm on the main surface of the glasssubstrate. Accordingly, the number of defects present on the surfacewith the arithmetic mean roughness (Ra) at the level in the vicinity of0.1 nm is very small.

Therefore, it is suitable as a substrate for manufacturing a magneticdisk having magnetic particles with a much smaller size, for example,having a recording density of 160 GB or more per disk, particularly 250GB or more per disk.

Further, according to the magnetic disk glass substrate manufacturingmethod of this invention, since the additive contained in the polishingliquid used in the polishing step is contained in the cleaning liquid,it is possible, in the cleaning step, to remove foreign matter (adheringmatter) from the surface of the glass substrate while maintaining theform of secondary aggregation of the polishing agent and maintaining theinteraction thereof with the surface of the glass substrate. Further,with the above-mentioned configuration, it is possible to easily removethe foreign matter by increasing the chemical affinity. As aconsequence, it is possible to manufacture a magnetic disk glasssubstrate which has very few defects present on its surface with anarithmetic mean roughness (Ra) at a level in the vicinity of 0.1 nm andthus is suitable as a substrate for a magnetic disk with high recordingdensity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows diagrams of a magnetic disk glass substrate according to anembodiment of this invention, wherein (a) is a side view, (b) is adiagram for explaining defects present on a surface thereof, and (c) isa diagram for explaining the roughness of the surface.

FIG. 2 is a diagram showing a schematic structure of an apparatusadapted to detect a defect on the magnetic disk glass substrate.

FIG. 3 is a diagram for explaining defects present on a surface of amagnetic disk glass substrate according to an embodiment of thisinvention.

FIG. 4 is a diagram for explaining a crack of a magnetic layer in amagnetic disk.

FIG. 5 is a diagram showing a schematic structure of an apparatusadapted to detect a defect on the magnetic disk glass substrate.

MODE FOR CARRYING OUT THE INVENTION

While developing a magnetic disk glass substrate that satisfies therequirement for improving the recording density which will increase moreand more in the future, the present inventors have found that, forexample, even using glass substrates with the same roughness and withthe same number of defects as a result of an inspection by a specificdefect inspection apparatus, there are differences in a reliability testand so on after the glass substrates are formed into magnetic disks.Then, as a result of diligently studying its reason, the presentinventors have found that, among the defects judged by the defectinspection apparatus, there are those fixed to the glass substrate andthose not fixed to the glass substrate and that the defects fixed to theglass substrate affect the reliability test and so on. Then, as a resultof further intensive studies for solving this problem, the presentinventors have found a method of significantly reducing defects fixed toa glass substrate, have found that it is possible to provide a magneticdisk substrate capable of achieving both a low roughness and few fixeddefects, and have completed this invention.

Further, the present inventors have found that if, among defects,particularly a concave defect of a specific size and depth is present ona glass substrate, the reliability of a magnet disk is adverselyaffected. Then, as a result of further intensive studies for solvingthis problem, the present inventors have found a method of significantlyreducing concave defects of a specific size and depth on a glasssubstrate, have found that it is possible to provide a magnetic diskglass substrate capable of achieving both a low roughness and fewconcave defects, and have completed this invention.

Hereinbelow, embodiments of this invention will be described in detailwith reference to the accompanying drawings.

Embodiment 1

FIG. 1 shows diagrams of a magnetic disk glass substrate according to anembodiment 1, wherein (a) is a side view, (b) is a diagram forexplaining defects present on a surface thereof, and (c) is a diagramfor explaining the roughness of the surface.

A magnetic disk glass substrate 1 shown in FIG. 1 is such that anarithmetic mean roughness (Ra) of a main surface of the glass substratemeasured using an atomic force microscope with a resolution of 256×256pixels in a 2 μm×2 μm square is 0.12 nm or less and, among defectsdetected to have a size of 0.1 μm or more and 0.3 μm or less uponirradiating light with a wavelength of 405 nm onto the glass substratewith a spot size of 5 μm and detecting scattered light from the glasssubstrate, the number of the defects fixed to the glass substrate is 1or less per 24 cm².

As shown in FIG. 1( b), defects present on a surface 1 a of the magneticdisk glass substrate 1 include adhering matter 1 b of the kind that canbe easily removed by cleaning, and a convex defect 1 c fixed to thesurface and a concave defect 1 d which either cannot be easily removedby cleaning. The defect intended in this embodiment is the convex defect1 c fixed to the surface or the concave defect 1 d such as a crackgenerated in glass substrate processing or a damage generated insubstrate fluidization/transfer. That is, the defect intended in thisembodiment is a convex defect or a concave defect (stationary defect)remaining without moving on the surface 1 a of the magnetic disk glasssubstrate 1 before and after cleaning under a condition of immersion for200 seconds in a dilute sulfuric acid solution adjusted to pH4. Thecleaning condition of the immersion for 200 seconds in the dilutesulfuric acid solution adjusted to pH4 is a condition that is sufficientfor removing the non-fixed adhering matter 1 b while maintaining thelevel of 0.12 nm or less of the surface roughness (arithmetic meanroughness (Ra)) of the surface 1 a of the magnetic disk glass substrate1 after the cleaning.

That is, in this embodiment, the stationary defect represents a defectwhose position on the glass substrate is not changed (a defect judgednot to have moved) before and after the glass substrate is immersed for200 seconds in the dilute sulfuric acid solution adjusted to pH4.Specifically, it is possible to specify a stationary defect byspecifying a position of a defect on the glass substrate using anoptical defect inspection apparatus, then specifying again a position ofthe defect on the glass substrate using the above-mentioned apparatusafter immersing the glass substrate for 200 seconds in the dilutesulfuric acid solution adjusted to pH4 and cleaning the glass substratewith water and IPA, and then comparing the positions of the defectbefore and after the cleaning.

It is considered that, with the arithmetic mean roughness (Ra) at thelevel of 0.12 nm or less, a defect that is not conventionally recognizedas a defect newly affects as a defect the properties of the magneticdisk glass substrate. Therefore, it is necessary to control the numberof defects that are detected by an apparatus capable of detecting adefect with a size of about 1 μm or less. In this embodiment, the numberof defects is the number of defects that are detected by such anapparatus capable of detecting a defect with a size of about 1 μm orless.

Herein, in this specification, the size of a defect represents the widthof a defect in a main surface direction (not a depth direction) of thesubstrate and, when a defect is not circular, the size of the defectrepresents the longer diameter thereof. For example, in FIG. 3, the sizeof a defect 21 d is given by W.

As the apparatus for inspecting a defect on the surface of the magneticdisk glass substrate 1, there is, for example, an apparatus having astructure shown in FIG. 2. The apparatus shown in FIG. 2 is an opticaldefect inspection apparatus (Optical Surface Analyzer) and comprises twolasers 11 and 12 and a detector 13 for detecting reflected light oflaser light. The laser 11 irradiates light having directivity onto theglass substrate 1 as a measurement object in a radial direction thereofwhile the laser 12 irradiates light having directivity onto the glasssubstrate 1 as the measurement object in a circumferential directionthereof. In such an apparatus, it is possible to detect a defect havinga length in the radial direction and a defect having a length in thecircumferential direction. Further, each laser is capable of separatinglaser light into its spectral components, i.e. capable of separatinglaser light into laser light in a direction perpendicular to the glasssubstrate 1 and laser light in a direction horizontal to the glasssubstrate 1. Suitable directivity of laser light depends on the kind ofa defect. Therefore, by separating the laser light into its spectralcomponents as described above, it is possible to accurately carry outdetection of various defects. Further, in the apparatus shown in FIG. 2,the laser size is as small as, for example, about 4 μm×5 μm, the laserwavelength is short, and the power is large so that the defect detectionsensitivity is high.

The magnetic disk glass substrate 1 according to this embodiment is suchthat the number of defects detected to have a size of 0.1 μm or more and0.3 μm or less is 1 or less per 24 cm² upon irradiating light with awavelength of 405 nm onto the substrate with a spot size of 5 μm anddetecting scattered light from the substrate using the apparatus shownin FIG. 2. The magnetic disk glass substrate having very few defectspresent on its surface with the arithmetic mean roughness (Ra) at thelevel in the vicinity of 0.1 nm as described above is suitable as asubstrate for a magnetic disk with high recording density.

The apparatus shown in FIG. 2 is one example. As long as an apparatus isof the type that irradiates light having directivity onto a glasssubstrate in circumferential and radial directions thereof and detects adefect based on reflected light from the glass substrate, it can besimilarly used in this embodiment.

In a hard disk drive (HDD) device incorporating a magnetic disk, withthe advance in device size reduction, the distance between the magneticdisk and a magnetic head has been reduced. As a consequence, withrespect to a convex defect among defects on the surface of the magneticdisk glass substrate 1 that cannot be easily removed by cleaning, aftera magnetic layer and so on are provided on the magnetic disk glasssubstrate 1 to form a magnetic disk, there is a possibility of theconvex defect exerting influence upon collision of the magnetic head.Therefore, it is preferable that particularly the convex defect be assmall as possible.

Specifically, a ratio (Ra/Rp) of an arithmetic mean roughness (Ra) to amaximum peak height (Rp) on a surface of a magnetic disk glass substrateis preferably 0.15 or more. Herein, as shown in FIG. 1( c), the maximumpeak height (Rp) represents a distance (height) between a mean referenceline (broken line in FIG. 1( c)) and a maximum peak portion.

By setting the ratio (Ra/Rp) of the arithmetic mean roughness (Ra) tothe maximum peak height (Rp) on the main surface to 0.15 or more, theflying stability of a magnetic head (particularly a DHF head) can befurther improved in a hard disk drive incorporating a magnetic diskmanufactured using such a glass substrate. Therefore, theabove-mentioned magnetic disk glass substrate can be suitably appliedeven to a hard disk drive with increased recording density.

Further, taking into account that the distance between the magnetic diskand the magnetic head has been reduced, the arithmetic mean roughness(Ra) is preferably uniform in a wide region of the magnetic disk glasssubstrate.

Specifically, as shown in FIG. 1( a), it is preferable that the magneticdisk glass substrate 1 have a disk shape with a hole 2 at the centerthereof and that, assuming that the distance from the center to theoutermost periphery is 100%, a difference (Ra_(O)-Ra_(I)) between anarithmetic mean roughness (Ra_(O)) of the main surface in a range of 80%or more and 90% or less from the center and an arithmetic mean roughness(Ra_(I)) of the main surface in a range of 10% or more and 20% or lessfrom the center be 0.01 nm or less.

With this structure, it is possible to reduce variation in surfaceroughness on the glass substrate surface and thus to further improve theflying stability of a magnetic head (particularly a DHF head).

As one example of a method of manufacturing this magnetic disk glasssubstrate, final polishing (herein, a second polishing process) may becarried out using a method of polishing the glass substrate surface byuniformly applying a force thereto. Specifically, for example, this canbe achieved using a planetary gear type polishing machine to polish aplurality of glass substrates while holding them between upper and lowersurface plates.

The magnetic disk glass substrate 1 according to this embodiment is anisotropic substrate. That is, the surface roughness (arithmetic meanroughness (Ra)) of the glass substrate in its circumferential directionand the surface roughness thereof in its radial direction are equal toeach other.

As a material of the magnetic disk glass substrate 1, there can be citeda multi-component glass such as an aluminosilicate glass, analuminoborosilicate glass, or a sodalime glass, a crystallized glass, orthe like. Particularly, the aluminosilicate glass is preferable becauseit can be easily processed and it can be increased in rigidity bychemical strengthening or the like.

The magnetic disk glass substrate 1 according to this embodiment hasmain surfaces and end faces and may be configured such that the mainsurfaces and the end faces are formed with a compressive stress layerand the depth of the compressive stress layer of each main surface isshallower than the depth of the compressive stress layer of each endface. The manufacture of the magnetic disk glass substrate of theabove-mentioned structure can be achieved, for example, by applyingchemical strengthening as ion exchange to the glass substrate and thenapplying a polishing process to both main surfaces of the substrate.

A magnetic disk is produced by forming at least a magnetic layer on themagnetic disk glass substrate 1 having the above-mentioned structure.That is, normally, a magnetic disk is manufactured by laminating anunderlayer, a magnetic layer, a protective layer, and a lubricatinglayer in this order on a magnetic disk glass substrate. The underlayerin the magnetic disk is properly selected depending on the magneticlayer.

Since the magnetic disk of this invention is free of specific defects,it can be suitably used particularly as a patterned medium in which atleast adjacent recording tracks are magnetically isolated from eachother.

Herein, the patterned medium is a magnetic recording medium in which aplurality of magnetic regions each serving as a recording bit unit areindependently formed in a nonmagnetic layer.

Embodiment 2

FIG. 3 is a diagram for explaining defects present on a surface of amagnetic disk glass substrate according to an embodiment 2.

A magnetic disk glass substrate 21 shown in FIG. 3 is such that anarithmetic mean roughness (Ra) of a main surface of the glass substratemeasured using an atomic force microscope with a resolution of 256×256pixels in a 2 μm×2 μm square is 0.12 nm or less and the number ofdefects detected to have a size of 0.1 μm or more and 0.6 μm or less inplan view and a depth of 0.5 nm or more and 2 nm or less is less than 10per 24 cm², the defects each being detected using a shift in wavelengthbetween incident light and reflected light upon irradiating and scanninghelium neon laser light with a wavelength of 632 nm on the main surfaceof the glass substrate.

As shown in FIG. 3, defects present on a surface 21 a of the magneticdisk glass substrate 21 include adhering matter 21 b of the kind thatcan be easily removed by cleaning, and a convex defect 21 c and aconcave defect 21 d which either cannot be easily removed by cleaning.In a conventional magnetic disk glass substrate, there is a micropit asa concave defect. This micropit has a size of several μm. Therefore, ifa magnetic layer is formed on the magnetic disk glass substrate in thestate where the micropit is present, the magnetic layer follows themicropit to form a concave portion and this concave portion forms amissing bit. Since this way the micropit conventionally forms themissing bit to cause a signal problem, it has been desired that themagnetic disk glass substrate has no micropit.

It has been found that when the increase in recording density advancesto reach a recording density of 160 GB or more per disk, particularly250 GB or more per disk, a very low surface roughness (Ra) at the levelin the vicinity of 0.1 nm is necessary and, simultaneously, concavedefects having a size of 1 μm or less, i.e. so-called nanopits, aredesirable to be as few as possible. This is because, as shown in FIG. 4,when a magnetic layer 23 is formed on the magnetic disk glass substrate21 in the state where a nanopit 22 is present, a crack 24 occursstarting from a magnetic layer portion on the nanopit. Then, if thecrack 24 occurs in the magnetic layer 23 in this manner, corrosion ofthe magnetic layer 23 proceeds from the crack portion. Accordingly, in asubstrate for a magnetic disk with a recording density of 250 GB or moreper disk, being free of nanopits is important in terms of thereliability of a magnetic layer. Therefore, the defect intended in thisembodiment is a concave defect having a size of 0.1 μm or more and 0.6μm or less in plan view and a depth of 0.5 nm or more and 2 nm or less(a so-called nanopit) and the technical idea of this embodiment is tocontrol the number of nanopits to be reduced in a magnetic disk glasssubstrate. In this embodiment, the number of defects is the number ofdefects that are detected by an apparatus capable of detecting such adefect having a size of 0.1 μm or more and 0.6 μm or less in plan viewand a depth of 0.5 nm or more and 2 nm or less.

As the apparatus for inspecting a defect on the surface of the magneticdisk glass substrate, there is, for example, an apparatus having astructure shown in FIG. 5. The apparatus shown in FIG. 5 is an opticaldefect inspection apparatus and is an apparatus using the laser Dopplertechnique. This apparatus comprises a laser 31 and a detector 32 fordetecting reflected light of laser light. The laser 31 irradiates andscans laser light on the magnetic disk glass substrate 21 as ameasurement object. Then, the detector 32 detects a nanopit based on ashift in wavelength between incident light and reflected light when thelaser light is irradiated and scanned on the magnetic disk glasssubstrate 21.

The magnetic disk glass substrate according to this embodiment is suchthat the number of defects detected to have a size of 0.1 μm or more and0.6 μm or less in plan view and a depth of 0.5 nm or more and 2 nm orless is less than 10 per 24 cm², wherein the defects are each detectedusing a shift in wavelength between incident light and reflected lightupon irradiating and scanning helium neon laser light with a wavelengthof 632 nm on the magnetic disk glass substrate by the use of theapparatus shown in FIG. 5. The glass substrate having very few defectspresent on its surface with the arithmetic mean roughness (Ra) at thelevel in the vicinity of 0.1 nm as described above is suitable as asubstrate for a magnetic disk with high recording density.

In the magnetic disk glass substrate 21 according to this embodiment, itis more preferable that a ratio (Ra/Rv) of an arithmetic mean roughness(Ra) to a maximum valley depth (Rv) on the main surface thereof be 0.15or more. With this structure, the flying stability of a magnetic head(particularly a DHF head) can be further improved in a hard disk driveincorporating a magnetic disk manufactured using such a glass substrate.Therefore, the above-mentioned magnetic disk glass substrate can besuitably applied even to a hard disk drive with increased recordingdensity.

It is more preferable that the magnetic disk glass substrate 21according to this embodiment have a disk shape with a hole at the centerthereof and that, assuming that the distance from the center to theoutermost periphery is 100%, a difference (Ra_(O)-Ra_(I)) between anarithmetic mean roughness (Ra_(O)) of the main surface in a range of 80%or more and 90% or less from the center and an arithmetic mean roughness(Ra_(I)) of the main surface in a range of 10% or more and 20% or lessfrom the center be 0.01 nm or less. With this structure, it is possibleto reduce variation in surface roughness on the glass substrate surfaceand thus to further improve the flying stability of a magnetic head(particularly a DHF head).

As one example of a method of manufacturing this magnetic disk glasssubstrate, final polishing (herein, a second polishing process) may becarried out using a method of polishing the glass substrate surface byuniformly applying a force thereto. Specifically, for example, this canbe achieved using a planetary gear type polishing machine to polish aplurality of glass substrates while holding them between upper and lowersurface plates.

The magnetic disk glass substrate 21 according to this embodiment hasmain surfaces and end faces and may be configured such that the mainsurfaces and the end faces are formed with a compressive stress layerand the depth of the compressive stress layer of each main surface isshallower than the depth of the compressive stress layer of each endface. The manufacture of the magnetic disk glass substrate of theabove-mentioned structure can be achieved, for example, by applyingchemical strengthening as ion exchange to the glass substrate and thenapplying a polishing process to both main surfaces of the glasssubstrate.

As a material of the magnetic disk glass substrate 21, there can becited a multi-component glass such as an aluminosilicate glass, analuminoborosilicate glass, or a sodalime glass, a crystallized glass, orthe like. Particularly, the aluminosilicate glass is preferable becauseit can be easily processed and it can be increased in rigidity bychemical strengthening or the like.

A magnetic disk is produced by forming at least a magnetic layer on themagnetic disk glass substrate having the above-mentioned structure. Thatis, normally, a magnetic disk is manufactured by laminating anunderlayer, a magnetic layer, a protective layer, and a lubricatinglayer in this order on a magnetic disk glass substrate. The underlayerin the magnetic disk is properly selected depending on the magneticlayer.

Since the magnetic disk of this invention is free of specific defects,it can be suitably used particularly as a patterned medium in which atleast adjacent recording tracks are magnetically isolated from eachother.

Embodiment 3

In this embodiment, a magnetic disk glass substrate manufacturing methodwill be described.

The magnetic disk glass substrate manufacturing method according to thisembodiment comprises a process of applying at least shaping and lappingto a glass substrate having a pair of main surfaces, a polishing processof polishing the main surfaces, and a cleaning process of cleaning theglass substrate after the polishing, wherein an additive contained in apolishing liquid for use in the polishing process is contained in acleaning liquid for use in the cleaning process.

More specifically, the manufacture of a magnetic disk glass substratecomprises (1) Shaping Process and First Lapping Process, (2) End PortionShaping Process (coring process for forming a hole and chamferingprocess for forming chamfered faces at end portions (outer peripheralend portion and inner peripheral end portion) (chamfered face formingprocess)), (3) End Face Polishing Process (outer peripheral end portionand inner peripheral end portion), (4) Second Lapping Process, and (5)Main Surface Polishing Process (first and second polishing processes)and Cleaning Process. Further, it is preferable to carry out a chemicalstrengthening process. It is possible to properly change the order ofthe respective processes, but in order to manufacture a magnetic diskglass substrate of this invention, it is preferable to carry out thechemical strengthening process and the second polishing process afterthe first polishing process.

In this embodiment, attention is paid to (5) Polishing Process of MainSurface Polishing Process and Cleaning Process among the above-mentionedprocesses. In the polishing process and the cleaning process, by causingan additive contained in a polishing liquid to be contained in acleaning liquid, it is possible to remove foreign matter (adheringmatter) from surfaces of a glass substrate while maintaining the form ofsecondary aggregation of the polishing agent and maintaining theinteraction thereof with the surfaces of the glass substrate. Further,with the above-mentioned configuration, it is possible to easily removethe foreign matter by increasing the chemical affinity. As aconsequence, it is possible to achieve a magnetic disk glass substratewhich has very few defects present on its surface with an arithmeticmean roughness (Ra) at a level in the vicinity of 0.1 nm and thus issuitable as a substrate for a magnetic disk with high recording density.

As the additive which is contained in the polishing liquid for use inthe polishing process and in the cleaning liquid for use in the cleaningprocess, there can be cited one containing at least one of carboxylicacids such as acetic acid, malic acid, oxalic acid, malonic acid,succinic acid, glycolic acid, citric acid, and tartaric acid, polyvalentamines such as ethylenediamine and diethylenetriamine, amino acids suchas glycine, alanine, serine, and aspartic acid, aminopolycarboxylicacids such as ethylenediaminetetraacetic acid and nitrilotriacetic acid,phosphonic acids such as hydroxyethylidene diphosphonic acid, methylenephosphonic acid, and hydroxyethane phosphonic acid (HEDP), phosphinicacids, phosphoric acids such as pyrophosphoric acid andtripolyphosphoric acid, pyrophosphoric acids, tripolyphosphoric acids,amino trimethylene phosphonic acids, and the like. Salts of the acidsexemplified above may also be used. Among them, salts of polyhydroxyacids such as sodium phosphate, monosodium dihydrogen phosphate,disodium monohydrogen phosphate, and potassium oxalate are morepreferable.

Among them, as one having a chelating action (a chelating agent), therecan be cited, for example, carboxylic acid such as oxalic acid, malonicacid, glycolic acid, citric acid, or tartaric acid, polyvalent aminesuch as ethylenediamine or diethylenetriamine, amino acid such asglycine, alanine, serine, or aspartic acid, aminopolycarboxylic acidsuch as ethylenediaminetetraacetic acid or nitrilotriacetic acid,phosphonic acid such as hydroxyethylidene diphosphonic acid or methylenephosphonic acid, phosphoric acid such as pyrophosphoric acid ortripolyphosphoric acid, or the like. As one having a dispersing action(a dispersant), there can be cited, for example, an anionic surfaceactive agent such as sulfo fatty acid ester, alkylbenzene sulfonic acid,alkyl sulfate, alkyl sulfuric acid triethanolamine, or alkyl ethersulfuric acid ester, a nonionic surface active agent such as fatty aciddiethanolamide, polyoxyethylene alkyl ether, or polyoxyethylene alkylphenyl ether, amine, or the like.

Taking into account the suppression of the surface roughness of theglass substrate, the foreign matter removal capability, excessive orinsufficient aggregation of the polishing agent, the influence onenvironment when the cleaning liquid is drained off, and so on, theadditive is preferably contained in the polishing liquid in a range of0.01 wt % or more and 10.0 wt % or less and more preferably in a rangeof 0.1 wt % or more and 5.0 wt % or less, while the additive ispreferably contained in the cleaning liquid in a range of 0.01 wt % ormore and 5.0 wt % or less and more preferably in a range of 0.1 wt % ormore and 3.0 wt % or less.

Next, a description will be given of Examples which were carried out forclarifying the effect of this invention.

EXAMPLE 1 RELATING TO EMBODIMENT 1

Hereinbelow, a description will be given of an Example about methods ofmanufacturing a magnetic disk glass substrate and a magnetic disk towhich this invention is applied. These magnetic disk glass substrate andmagnetic disk are manufactured as a magnetic disk having a predeterminedshape, such as a 3.5-inch disk (φ89 mm) or a 2.5-inch disk (φ65 mm).

(1) First Lapping Process

In the magnetic disk glass substrate manufacturing method according tothis Example, first, lapping (grinding) is applied to surfaces of aplate-like glass to obtain a glass base member, then this glass basemember is cut, thereby cutting out a glass disk. As the plate-likeglass, one of various plate-like glasses can be used. This plate-likeglass can be manufactured by a known manufacturing method such as apress method, a float method, a downdraw method, a redraw method, or afusion method using, for example, a molten glass as a material. If thepress method is used among them, the plate-like glass can bemanufactured at low cost. As a material property of the plate-likeglass, use can be made of an amorphous glass or a glass ceramic(crystallized glass). As a material of the plate-like glass, use can bemade of an aluminosilicate glass, a sodalime glass, a borosilicateglass, or the like. Particularly as the amorphous glass, thealuminosilicate glass can be preferably used because it can bechemically strengthened and it can provide a magnetic disk glasssubstrate excellent in main surface flatness and in substrate strength.

In this Example, a molten aluminosilicate glass was molded into a diskshape by direct pressing using upper, lower, and drum molds, therebyobtaining an amorphous plate-like glass. As the aluminosilicate glass,use was made of a glass containing, as main components, SiO₂: 58 wt % to75 wt %, Al₂O₃: 5 wt % to 23 wt %, Li₂O: 3 wt % to 10 wt %, and Na₂O: 4wt % to 13 wt %.

Then, lapping was applied to both main surfaces of the plate-like glass,thereby obtaining a disk-shaped glass base member. The lapping wascarried out using a double-side lapping machine employing a planetarygear mechanism with the use of alumina-based free abrasive particles.Specifically, the lapping was carried out by pressing lapping surfaceplates onto both surfaces of the plate-like glass from the upper andlower sides, supplying a grinding liquid containing the free abrasiveparticles onto the main surfaces of the plate-like glass, and relativelymoving them to each other. By this lapping, the glass base member havingthe flat main surfaces was obtained.

(2) Shaping Process (Coring, Chamfering)

Then, using a cylindrical diamond drill, an inner hole was formed at acentral portion of the glass substrate, thereby obtaining an annularglass substrate (coring). Then, grinding was applied to an innerperipheral end face and an outer peripheral end face using diamondgrindstones, thereby carrying out predetermined chamfering (chamfering).

(3) Second Lapping Process

Then, second lapping was applied to both main surfaces of the obtainedglass substrate in the same manner as in the first lapping process. Bycarrying out this second lapping process, fine irregularities formed onthe main surfaces in the cutting-out process or an end face polishingprocess as a preceding process can be removed in advance, so that itbecomes possible to complete a subsequent polishing process of the mainsurfaces in a short time.

(4) End Face Polishing Process

Then, the outer peripheral end face and the inner peripheral end face ofthe glass substrate were mirror-polished by a brush polishing method. Inthis event, as polishing abrasive particles, use was made of a slurry(free abrasive particles) containing cerium oxide abrasive particles.

Then, the glass substrate having been subjected to the end facepolishing process was washed with water. By this end face polishingprocess, the end faces of the glass substrate were finished to a mirrorsurface state that can prevent precipitation of sodium and potassium.

(5) First Polishing Process

A first polishing process was first carried out as a main surfacepolishing process. This first polishing process mainly aims to removecracks or strains remaining on the main surfaces during theabove-mentioned lapping processes. In this first polishing process, themain surfaces were polished using a double-side polishing machine havinga planetary gear mechanism with the use of a hard resin polisher. Ceriumoxide abrasive particles were used as a polishing agent.

The glass substrate having been subjected to the first polishing processwas immersed in respective cleaning baths of neutral detergent, purewater, and IPA (isopropyl alcohol) in turn so as to be cleaned.

(6) Chemical Strengthening Process

Then, chemical strengthening (ion exchange) was applied to the glasssubstrate having been subjected to the above-mentioned end facepolishing process and first main surface polishing process. The chemicalstrengthening was carried out by preparing a chemical strengtheningsolution in the form of a mixture of potassium nitrate (60%) and sodiumnitrate (40%), heating this chemical strengthening solution to 400° C.and preheating the cleaned glass substrate to 300° C., and immersing itin the chemical strengthening solution for about 3 hours. In order tochemically strengthen the entire surfaces of the glass substrate, theimmersion was carried out in the state where a plurality of glasssubstrates were placed in a holder so as to be held at their end faces.

By carrying out the immersion in the chemical strengthening solution asdescribed above, lithium ions and sodium ions in a surface layer of theglass substrate are replaced by sodium ions and potassium ions in thechemical strengthening solution, respectively, so that the glasssubstrate is strengthened. The thickness of a compressive stress layerformed at the surface layer of the glass substrate was about 100 μm to200 μm.

The glass substrate having been subjected to the chemical strengtheningwas immersed in a water bath at 20° C. so as to be rapidly cooled andwas maintained for about 10 minutes. Then, the rapidly cooled glasssubstrate was immersed in concentrated sulfuric acid heated to about 40°C., so as to be cleaned. Further, the glass substrate having beensubjected to the sulfuric acid cleaning was immersed in respectivecleaning baths of pure water and IPA in turn so as to be cleaned.

(7) Second Polishing Process

Then, a second polishing process was carried out as a main surfacepolishing process. This second polishing process aims to finish the mainsurfaces to a mirror surface state. In this second polishing process,the main surfaces were mirror-polished using a double-side polishingmachine having a planetary gear mechanism with the use of a soft resinfoam polisher. As a polishing agent, use was made of a slurry usingcolloidal silica abrasive particles (average particle size 5 nm to 80nm) finer than the cerium oxide abrasive particles used in the firstpolishing process.

The polishing was carried out by setting the pH of the slurry to 2. Inthis event, the polishing was carried out by adding an additivecontaining acetic acid and acetate to the slurry. This is forcontrolling the pH of the slurry to be constant during the polishingprocess. As the above-mentioned slurry (polishing liquid), use was madeof one in which 0.5 wt % citric acid was added as an additive to a mixedsolution obtained by adding the above-mentioned colloidal silicaparticles to ultrapure water.

(8) Cleaning Process

The glass substrate having been subjected to the second polishingprocess was immersed in respective cleaning baths of acid cleaning,alkaline cleaning, pure water, and IPA in turn so as to be cleaned. Anultrasonic wave was applied to each cleaning bath.

In the acid cleaning, the same additive as that added in theabove-mentioned second polishing process was added as an additive forthe acid cleaning. Specifically, the acid cleaning was carried out usingan acid solution in which citric acid was adjusted to 0.15 wt %. This isfor efficiently removing the slurry fixed to the substrate by puttingthe same component as that contained in the slurry into the cleaningliquid. As a consequence, it is possible to reduce particles fixed tothe glass substrate.

By carrying out the first lapping process, the cutting-out process, thesecond lapping process, the end face polishing process, the firstpolishing process, the chemical strengthening process, and the secondpolishing process as described above, there was obtained a flat, smooth,and high-rigidity magnetic disk glass substrate.

COMPARATIVE EXAMPLE 1 RELATING TO EMBODIMENT 1

A glass substrate was manufactured in the same manner as in Example 1relating to the embodiment 1 except that the additive contained in apolishing liquid used in the polishing process was not contained in acleaning liquid used in the cleaning process.

COMPARATIVE EXAMPLE 2 RELATING TO EMBODIMENT 1

A glass substrate was manufactured in the same manner as in Example 1relating to the embodiment 1 except that the content of citric acid in acleaning liquid used in the cleaning process was adjusted to 0.005 wt %.

(Defect Inspection 1)

The glass substrates obtained in the Example and the ComparativeExamples were each subjected to a defect inspection using the opticaldefect inspection apparatus (manufactured by KLA-Tencor Corporation,trade name: OSA6100) shown in FIG. 2. In this event, a region of 15 mmto 31.5 mm from the center of the glass substrate was measured under themeasurement conditions that the laser power was set to 25 mW, the laserwavelength to 405 nm, and the laser spot size to 5 μm. Table 1 shows thenumber of fixed defects (per 24 cm²) among defects detected to have asize of 0.1 μm or more and 0.3 μm or less.

(Surface Measurement of Glass Substrate)

The glass substrates obtained in the Example and the ComparativeExamples were each measured using an atomic force microscope with aresolution of 256×256 pixels in a 2 μm×2 μm square to thereby obtain asurface roughness (arithmetic mean roughness (Ra)) thereof. The resultsare shown in Table 1.

Further, assuming that the distance from the center of the glasssubstrate to the outermost periphery thereof is 100%, an arithmetic meanroughness (Ra_(O)) of the main surface in a range of 80% or more and 90%or less from the center and an arithmetic mean roughness (Ra_(I)) of themain surface in a range of 10% or more and 20% or less from the centerwere measured, and then a difference (Ra_(O)-Ra_(I)) therebetween wasderived. The results are shown in Table 1.

A maximum peak height (Rp) and an arithmetic mean roughness (Ra) on thesurface of each glass substrate were measured and then a ratio (Ra/Rp)of the arithmetic mean roughness (Ra) to the maximum peak height (Rp)was derived. The results are shown in Table 1.

Then, an adhesive layer, a soft magnetic layer, a pre-underlayer, anunderlayer, a nonmagnetic granular layer, a first magnetic recordinglayer, a second magnetic recording layer, an auxiliary recording layer,a protective layer, and a lubricating layer were laminated in this orderon each of the glass substrates obtained in the Example and theComparative Examples, thereby manufacturing magnetic disks.

Specifically, using an evacuated film forming apparatus, the layers fromthe adhesive layer to the auxiliary recording layer were formed insequence on each disk substrate in an Ar atmosphere by a DC magnetronsputtering method. The adhesive layer was made of CrTi. The softmagnetic layer was such that a Ru spacer layer was interposed between afirst soft magnetic layer and a second soft magnetic layer each made ofFeCoTaZr. The composition of the pre-underlayer was a NiW alloy with anfcc structure. The underlayer was such that a second underlayer (Ru)formed in high-pressure Ar was laminated on a first underlayer (Ru)formed in low-pressure Ar. The composition of the nonmagnetic granularlayer was nonmagnetic CoCr—SiO₂. The composition of the first magneticrecording layer was CoCrPt—Cr₂O₃ and the composition of the secondmagnetic recording layer was CoCrPt—SiO₂—TiO₂. The composition of theauxiliary recording layer was CoCrPtB. The medium protective layer wasformed by a CVD method using C₂H₄ and further by carrying out nitridingto introduce nitrogen into a surface thereof in the same chamber. Thelubricating layer was formed by a dip coating method using PFPE.

In this event, since the amount of defects (amount of contamination) onthe main surface of each glass substrate was at a very low level, theorientations of magnetic particles by sputtering were aligned so that itwas possible to form the magnetic layers capable of high densityrecording. A durability test was conducted for the magnetic disks thusobtained.

(Durability Test)

The durability test was conducted by mounting the magnetic disk in anLUL (load/unload) type HDD device. Specifically, in the state where themagnetic disk and a DFH head having a giant magnetoresistive effectreproducing element (GMR element) were installed in the magneticrecording device, the durability test was conducted by carrying outload/unload tests in a predetermined number of times (2,000,000 times)at a head flying height of 6 nm. The results are shown in Table 1.

TABLE 1 Number of AFM-Ra/ Ra_(O)-Ra_(I)/ Durability Test Defects nmRa/Rp nm Result Example 1 1 or less 0.11 0.14 0.004 2,000,000 times L/ULOK Comparative 6 0.12 0.17 0.006   400,000 times Example 1 L/UL NGComparative 3 0.12 0.15 0.005   800,000 times Example 2 L/UL NG

EXAMPLE 1 RELATING TO EMBODIMENT 2

Hereinbelow, a description will be given of an Example about methods ofmanufacturing a magnetic disk glass substrate and a magnetic disk towhich this invention is applied. These magnetic disk glass substrate andmagnetic disk are manufactured as a magnetic disk having a predeterminedshape, such as a 3.5-inch disk (φ89 mm) or a 2.5-inch disk (φ65 mm).

(1) First Lapping Process

In the magnetic disk glass substrate manufacturing method according tothis Example, first, lapping (grinding) is applied to surfaces of aplate-like glass to obtain a glass base member, then this glass basemember is cut, thereby cutting out a glass disk. As the plate-likeglass, one of various plate-like glasses can be used. This plate-likeglass can be manufactured by a known manufacturing method such as apress method, a float method, a downdraw method, a redraw method, or afusion method using, for example, a molten glass as a material. If thepress method is used among them, the plate-like glass can bemanufactured at low cost. As a material property of the plate-likeglass, use can be made of an amorphous glass or a glass ceramic(crystallized glass). As a material of the plate-like glass, use can bemade of an aluminosilicate glass, a sodalime glass, a borosilicateglass, or the like. Particularly as the amorphous glass, thealuminosilicate glass can be preferably used because it can bechemically strengthened and it can provide a magnetic disk glasssubstrate excellent in main surface flatness and in substrate strength.

In this Example, a molten aluminosilicate glass was molded into a diskshape by direct pressing using upper, lower, and drum molds, therebyobtaining an amorphous plate-like glass. As the aluminosilicate glass,use was made of a glass containing, as main components, SiO₂: 58 wt % to75 wt %, Al₂O₃: 5 wt % to 23 wt %, Li₂O: 3 wt % to 10 wt %, and Na₂O: 4wt % to 13wt %.

Then, lapping was applied to both main surfaces of the plate-like glass,thereby obtaining a disk-shaped glass base member. The lapping wascarried out using a double-side lapping machine employing a planetarygear mechanism with the use of alumina-based free abrasive particles.Specifically, the lapping was carried out by pressing lapping surfaceplates onto both surfaces of the plate-like glass from the upper andlower sides, supplying a grinding liquid containing the free abrasiveparticles onto the main surfaces of the plate-like glass, and relativelymoving them to each other. By this lapping, the glass base member havingthe flat main surfaces was obtained.

(2) Shaping Process (Coring, Chamfering)

Then, using a cylindrical diamond drill, an inner hole was formed at acentral portion of the glass substrate, thereby obtaining an annularglass substrate (coring). Then, grinding was applied to an innerperipheral end face and an outer peripheral end face using diamondgrindstones, thereby carrying out predetermined chamfering (chamfering).

(3) Second Lapping Process

Then, second lapping was applied to both main surfaces of the obtainedglass substrate in the same manner as in the first lapping process. Bycarrying out this second lapping process, fine irregularities formed onthe main surfaces in the cutting-out process or an end face polishingprocess as a preceding process can be removed in advance, so that itbecomes possible to complete a subsequent polishing process of the mainsurfaces in a short time.

(4) End Face Polishing Process

Then, the outer peripheral end face and the inner peripheral end face ofthe glass substrate were mirror-polished by a brush polishing method. Inthis event, as polishing abrasive particles, use was made of a slurry(free abrasive particles) containing cerium oxide abrasive particles.

Then, the glass substrate having been subjected to the end facepolishing process was washed with water. By this end face polishingprocess, the end faces of the glass substrate were finished to a mirrorsurface state that can prevent precipitation of sodium and potassium.

(5) First Polishing Process

A first polishing process was first carried out as a main surfacepolishing process. This first polishing process mainly aims to removecracks or strains remaining on the main surfaces during theabove-mentioned lapping processes. In this first polishing process, themain surfaces were polished using a double-side polishing machine havinga planetary gear mechanism with the use of a hard resin polisher. Ceriumoxide abrasive particles were used as a polishing agent.

The glass substrate having been subjected to the first polishing processwas immersed in respective cleaning baths of neutral detergent, purewater, and IPA (isopropyl alcohol) in turn so as to be cleaned.

(6) Chemical Strengthening Process

Then, chemical strengthening (ion exchange) was applied to the glasssubstrate having been subjected to the above-mentioned end facepolishing process and first main surface polishing process. The chemicalstrengthening was carried out by preparing a chemical strengtheningsolution in the form of a mixture of potassium nitrate (60%) and sodiumnitrate (40%), heating this chemical strengthening solution to 400° C.and preheating the cleaned glass substrate to 300° C., and immersing itin the chemical strengthening solution for about 3 hours. In order tochemically strengthen the entire surfaces of the glass substrate, theimmersion was carried out in the state where a plurality of glasssubstrates were placed in a holder so as to be held at their end faces.

By carrying out the immersion in the chemical strengthening solution asdescribed above, lithium ions and sodium ions in a surface layer of theglass substrate are replaced by sodium ions and potassium ions in thechemical strengthening solution, respectively, so that the glasssubstrate is strengthened. The thickness of a compressive stress layerformed at the surface layer of the glass substrate was about 100 μm to200 μm.

The glass substrate having been subjected to the chemical strengtheningwas immersed in a water bath at 20° C. so as to be rapidly cooled andwas maintained for about 10 minutes. Then, the rapidly cooled glasssubstrate was immersed in concentrated sulfuric acid heated to about 40°C., so as to be cleaned. Further, the glass substrate having beensubjected to the sulfuric acid cleaning was immersed in respectivecleaning baths of pure water and IPA in turn so as to be cleaned.

(7) Second Polishing Process

Then, a second polishing process was carried out as a main surfacepolishing process. This second polishing process aims to finish the mainsurfaces to a mirror surface state. In this second polishing process,the main surfaces were mirror-polished using a double-side polishingmachine having a planetary gear mechanism with the use of a soft resinfoam polisher. As a polishing agent, use was made of a slurry usingcolloidal silica abrasive particles (average particle size 5 nm to 80nm) finer than the cerium oxide abrasive particles used in the firstpolishing process.

The polishing was carried out by setting the pH of the slurry to 2. Inthis event, the polishing was carried out by adding an additivecontaining acetic acid and acetate to the slurry. This is forcontrolling the pH of the slurry to be constant during the polishingprocess. As the above-mentioned slurry (polishing liquid), use was madeof one in which 0.5 wt % citric acid was added as an additive to a mixedsolution obtained by adding the above-mentioned colloidal silicaparticles to ultrapure water.

(8) Cleaning Process

The glass substrate having been subjected to the second polishingprocess was immersed in respective cleaning baths of acid cleaning,alkaline cleaning, pure water, and IPA in turn so as to be cleaned. Anultrasonic wave was applied to each cleaning bath.

In the acid cleaning, the same additive as that added in theabove-mentioned second polishing process was added as an additive forthe acid cleaning. Specifically, the acid cleaning was carried out usingan acid solution in which citric acid was adjusted to 0.15 wt %. This isfor efficiently removing the slurry fixed to the substrate by puttingthe same component as that contained in the slurry into the cleaningliquid. As a consequence, it is possible to reduce particles fixed tothe glass substrate.

By carrying out the first lapping process, the cutting-out process, thesecond lapping process, the end face polishing process, the firstpolishing process, the chemical strengthening process, and the secondpolishing process as described above, there was obtained a flat, smooth,and high-rigidity magnetic disk glass substrate.

COMPARATIVE EXAMPLE 1 RELATING TO EMBODIMENT 2

A glass substrate was manufactured in the same manner as in Example 1relating to the embodiment 2 except that the additive contained in apolishing liquid used in the polishing process was not contained in acleaning liquid used in the cleaning process and that the secondpolishing process was carried out before the chemical strengtheningprocess.

EXAMPLES 2-3 AND COMPARATIVE EXAMPLES 2-6 RELATING TO EMBODIMENT 2

Magnetic disk glass substrates were respectively manufactured byvariously changing the polishing conditions and the cleaning conditions.

(Defect Inspection 2)

The glass substrates were each subjected to a defect inspection based onthe laser Doppler technique using the apparatus shown in FIG. 5. In thisevent, ThoT Model 42000 (manufactured by ThoT Technologies, Inc.) wasused as the inspection apparatus. The number of defects having a size of0.1 μm or more and 0.6 μm or less in plan view and a depth of 0.5 nm ormore and 2 nm or less was derived. The results are shown in Table 2.

(Surface Measurement of Glass Substrate)

The glass substrates obtained in the Examples and the ComparativeExamples were each measured using an atomic force microscope with aresolution of 256×256 pixels in a 2 μm×2 μm square to thereby obtain asurface roughness (arithmetic mean roughness (Ra)) thereof. The resultsare shown in Table 2.

Further, assuming that the distance from the center of the glasssubstrate to the outermost periphery thereof is 100%, an arithmetic meanroughness (Ra_(O)) of the main surface in a range of 80% or more and 90%or less from the center and an arithmetic mean roughness (Ra_(I)) of themain surface in a range of 10% or more and 20% or less from the centerwere measured, and then a difference (Ra_(O)-Ra_(I)) therebetween wasderived. The results are shown in Table 2.

A maximum valley depth (Rv) and an arithmetic mean roughness (Ra) on thesurface of each glass substrate were measured and then a ratio (Ra/Rv)of the arithmetic mean roughness (Ra) to the maximum valley depth (Rv)was derived. The results are shown in Table 2.

Then, an adhesive layer, a soft magnetic layer, a pre-underlayer, anunderlayer, a nonmagnetic granular layer, a first magnetic recordinglayer, a second magnetic recording layer, an auxiliary recording layer,a protective layer, and a lubricating layer were laminated in this orderon each of the glass substrates obtained in the Examples and theComparative Examples, thereby manufacturing magnetic disks.

Specifically, using an evacuated film forming apparatus, the layers fromthe adhesive layer to the auxiliary recording layer were formed insequence on each disk substrate in an Ar atmosphere by a DC magnetronsputtering method. The adhesive layer was made of CrTi. The softmagnetic layer was such that a Ru spacer layer was interposed between afirst soft magnetic layer and a second soft magnetic layer each made ofFeCoTaZr. The composition of the pre-underlayer was a NiW alloy with anfcc structure. The underlayer was such that a second underlayer (Ru)formed in high-pressure Ar was laminated on a first underlayer (Ru)formed in low-pressure Ar. The composition of the nonmagnetic granularlayer was nonmagnetic CoCr—SiO₂. The composition of the first magneticrecording layer was CoCrPt—Cr₂O₃ and the composition of the secondmagnetic recording layer was CoCrPt—SiO₂—TiO₂. The composition of theauxiliary recording layer was CoCrPtB. The medium protective layer wasformed by a CVD method using C₂H₄ and further by carrying out nitridingto introduce nitrogen into a surface thereof in the same chamber. Thelubricating layer was formed by a dip coating method using PFPE.

(LUL (Load/Unload) Test)

The above-mentioned magnetic disk and a DFH head having a giantmagnetoresistive effect reproducing element (GMR element) were installedin a magnetic recording device. Then, with a flying height of 10 nmduring flying of the magnetic head, load/unload operations of the headwere repeated in a high-temperature, high-humidity environment of 70° C.and 80% RH in the magnetic recording device.

(Corrosion Inspection)

The obtained magnetic disks were left in a high-temperature,high-humidity environment of 70° C. and 80% RH for 120 hours and thentaken out. Then, it was inspected whether or not corrosion occurred on asurface of each magnetic disk, by a visual inspection under ahigh-intensity halogen lamp and an inspection using an opticalmicroscope with 50 times magnification.

Evaluation criteria were defined as follows.

(Number of Luminescent Spots per 1 cm²)

=0

◯=1 to 2

Δ=3 to 5

×=6 to 10

××=11 or more

TABLE 2 Number of Durability Test Corrosion Defects AFM-Ra/nm Ra/RpRa_(O)-Ra_(I)/nm Result Test Example 1 5 0.11 0.15 0.004 2,000,000 timesL/UL   • OK Example 2 9 0.12 0.18 0.005 2,000,000 times L/UL   • OKExample 3 9 0.12 0.14 0.005 1,800,000 times L/UL   ∘ NG Comparative 530.17 0.13 0.006 200,000 times L/UL xx Example 1 NG Comparative 40 0.150.14 0.005 200,000 times L/UL xx Example 2 NG Comparative 25 0.19 0.120.006 300,000 times L/UL x Example 3 NG Comparative 11 0.21 0.11 0.005300,000 times L/UL Δ Example 4 NG Comparative 8 0.17 0.15 0.005 500,000times L/UL ∘ Example 5 NG Comparative 14 0.12 0.15 0.005 800,000 timesL/UL ∘ Example 6 NG Reference 9 0.12 0.14 0.012 1,500,000 times L/UL   ∘Example NG

As described above, according to this invention, it is possible toobtain a magnetic disk glass substrate having very few defects presenton its surface with an arithmetic mean roughness (Ra) at a level in thevicinity of 0.1 nm and thus to achieve the magnetic disk glass substratewhich is suitable as a substrate for a magnetic disk with a highrecording density of 160 GB or more per disk, particularly 250 GB ormore per disk.

A magnetic disk glass substrate manufacturing method of this inventionis preferably configured such that it comprises a process of applying atleast shaping and lapping to a disk base member having a main surface, apolishing process of polishing the main surface, and a cleaning processof cleaning the disk base member after the polishing, wherein anadditive contained in a polishing liquid for use in the polishingprocess is contained in a cleaning liquid for use in the cleaningprocess. According to this method, by causing the additive contained inthe polishing liquid for use in the polishing process to be contained inthe cleaning liquid for use in the cleaning process, it is possible toobtain a magnetic disk glass substrate having very few defects presenton its surface with an arithmetic mean roughness (Ra) at a level in thevicinity of 0.1 nm and thus to achieve the magnetic disk glass substratewhich is suitable as a substrate for a magnetic disk with a highrecording density of 160 GB or more per disk, particularly 250 GB ormore per disk.

This invention is not limited to the above-mentioned embodiments and canbe carried out by appropriately changing them. For example, the numberof components, the sizes, the processing sequences, and so on in theabove-mentioned embodiments are only examples and this invention can becarried out by changing them in various ways within a range capable ofexhibiting the effect of this invention. Other than those, thisinvention can be carried out with appropriate changes within a range notdeparting from the object of this invention.

Description of Symbols

1 magnetic disk glass substrate

1 a surface

1 b adhering matter

1 c convex defect

1 d concave defect

2 hole

11, 12 laser

13 detector

21 magnetic disk glass substrate

21 a surface

21 b adhering matter

21 c convex defect

21 d concave defect

22 nanopit

23 magnetic layer

24 crack

31 laser

32 detector

1. A magnetic disk glass substrate, wherein an arithmetic mean roughness(Ra) of a main surface of the glass substrate measured using an atomicforce microscope with a resolution of 256×256 pixels in a 2 μm×2 μmsquare is 0.12 nm or less and, among defects detected to have a size of0.1 μm or more and 0.3 μm or less upon irradiating light with awavelength of 405 nm onto the glass substrate with a spot size of 5 μmand detecting scattered light from the glass substrate, the number ofthe defects fixed to the glass substrate is 1 or less per 24 cm².
 2. Themagnetic disk glass substrate according to claim 1, wherein a ratio(Ra/Rp) of the arithmetic mean roughness (Ra) to a maximum peak height(Rp) on the main surface of the glass substrate is 0.15 or more.
 3. Amagnetic disk glass substrate, wherein an arithmetic mean roughness (Ra)of a main surface of the glass substrate measured using an atomic forcemicroscope with a resolution of 256×256 pixels in a 2 μm×2 μm square is0.12 nm or less and the number of defects detected to have a size of 0.1μm or more and 0.6 μm or less in plan view and a depth of 0.5 nm or moreand 2 nm or less is less than 10 per 24 cm², the defects each beingdetected using a shift in wavelength between incident light andreflected light upon irradiating and scanning helium neon laser lightwith a wavelength of 632 nm on the main surface of the glass substrate.4. The magnetic disk glass substrate according to claim 3, wherein aratio (Ra/Rv) of the arithmetic mean roughness (Ra) to a maximum valleydepth (Rv) on the main surface of the glass substrate is 0.15 or more.5. The magnetic disk glass substrate according to claim 1, wherein theglass substrate has a disk shape with a hole at a center thereof and,assuming that a distance from the center to an outermost periphery is100%, a difference (Ra_(O)-Ra_(I)) between an arithmetic mean roughness(Ra_(O)) of the main surface in a range of 80% or more and 90% or lessfrom the center and an arithmetic mean roughness (Ra_(I)) of the mainsurface in a range of 10% or more and 20% or less from the center is0.01 nm or less.
 6. The magnetic disk glass substrate according to claim1, wherein the glass substrate has the main surface and an end face, themain surface and the end face each have a compressive stress layer, andthe compressive stress layer of the main surface has a depth which isshallower than that of the compressive stress layer of the end face. 7.A magnetic disk, wherein at least a magnetic layer is formed on themagnetic disk glass substrate according to claim
 1. 8. The magnetic diskaccording to claim 7, wherein the magnetic disk is a patterned medium inwhich at least adjacent recording tracks are magnetically isolated fromeach other.
 9. A magnetic disk glass substrate manufacturing methodcomprising: polishing a main surface of a glass substrate using apolishing liquid containing an additive; and cleaning the glasssubstrate polished, using a cleaning liquid containing the additive. 10.The magnetic disk glass substrate manufacturing method according toclaim 9, wherein the additive contains at least one of carboxylic acid,polyvalent amine, amino acid, aminopolycarboxylic acid, phosphonic acid,phosphinic acid, phosphoric acid, pyrophosphoric acid, tripolyphosphoricacid, amino trimethylene phosphonic acid, and salts thereof.
 11. Themagnetic disk glass substrate manufacturing method according to claim10, wherein the additive is contained in the polishing liquid in a rangeof 0.01 wt % or more and 10.0 wt % or less and is contained in thecleaning liquid in a range of 0.01 wt % or more and 5.0 wt % or less.12. The magnetic disk glass substrate manufacturing method according toclaim 11, wherein the additive is contained in the polishing liquid in arange of 0.1 wt % or more and 5.0 wt % or less and is contained in thecleaning liquid in a range of 0.1 wt % or more and 3.0 wt % or less.